There were a few early Dells that sported a
horizontal "Mapback," obviously because that was the best way to orient the map.

Universal Publishing experimented with
horizontal front cover art with their Beacon and Softcover Library books as
well. And no, I am most definitely NOT going to crack a joke about that being
the best way to orient some people.

Here's a Ballantine Sci-Fi Cover

And, just to show that it's still in vogue,
here's a paperback cover from the 1990's from contributor Ralph Carlson.